In recent years, a reduction of fuel consumption of an internal combustion engine has been important. A method of improving the fuel efficiency is increasingly employed in which a large amount of EGR (Exhaust Gas Recirculation) gas is introduced in a light-loaded operation region for the purpose of low fuel consumption on the basis of the fact that pumping loss is reduced by introducing an EGR gas. However, in such a case, an inert gas is increased to reduce a ratio of the air which is newly introduced into a cylinder of the internal combustion engine, and accordingly a ratio of an appropriate air-fuel mixture is reduced around an ignition plug. Therefore, a secure ignition and a stable combustion are hardly achieved in a short-term discharge, and the internal combustion engine comes to be in an unstable state due to an incorrect combustion.
In an ignition device for an internal combustion engine, a state of the air-fuel mixture deviates in the vicinity of the ignition plug so as to cause a state where the air-fuel ratio is reduced or a state where an inert gas such as an EGR gas is mixed. In such a state, it is considered as a very important technique that an unstable ignitionability of the air-fuel mixture is improved in a short-term discharge and the fuel efficiency of the internal combustion engine is improved by improving the ignitionability and by making the combustion stable. For example, there is a well-known technique “Ignition device of overlap discharge type for internal combustion engine” (PTL 1) as the related art.